A boost converter generally increases a power supply voltage to drive a control circuit from an output voltage. When the output voltage is lower than a voltage capable of driving the control circuit, however, boosting the input voltage by using a startup circuit is needed to increases the power supply voltage capable of driving the control circuit. When the control circuit starts to operate by the startup circuit, the output voltage rises. When the output voltage exceeds the voltage capable of driving the control circuit, the control circuit starts to operate by the output voltage. Since the output voltage of the startup circuit becomes unnecessary at this point, operation of the startup circuit can then be stopped. Stopping the startup circuit suppresses power consumption and raises the efficiency of the whole boost converter.
The input voltage is input to the startup circuit via a switch for the startup circuit. Using a normally-on-type switch which is in an on-state even when a gate does not increase a voltage can make the input voltage input to the startup circuit. A negative voltage is then necessary to turn off the normally-on-type switch. In a background boost converter, an internal oscillator of the startup circuit is used to apply the negative voltage. The internal oscillator outputs a voltage oscillating positively and negatively around ground potential. The output is rectified and retained. When the control circuit is driven by the output voltage, the negative voltage is capable of turning the switch for the startup circuit off. When the switch for the startup circuit is turned off, the startup circuit is stopped by lack of the input voltage.
The negative voltage capable of turning the switch for the startup circuit off is held by a capacitor. The negative voltage held by the capacitor may change over time, however, so it is necessary to refresh the negative voltage. In a background technique, it is necessary to drive the oscillator of the startup circuit for refreshing the negative voltage held by the capacitor again. Therefore, the oscillator of the startup circuit consumes electric power, the power consumption of the boost converter increases, and then a problem of a decrease in efficiency of the boost converter arises.